Abstract A commercial preparation of glucose oxidase is shown to oxidize d -glucose, d -galactose, d -mannose, and d -xylose at relative rates which are pH dependent. The product of these reactions is the corresponding glyconic acid which in each case is identified as the crystalline γ-lactone and amide. Partial heat inactivation, fractional precipitation, and pH-optima studies indicate that the oxidation of each sugar is catalyzed by its respective oxidase. The enzyme preparation also showed amylase but not maltase activity.